Thermodynamics of binding of a sulfonamide inhibitor to metal-mutated carbonic anhydrase as studied by affinity capillary electrophoresis

By affinity capillary electrophoresis (ACE), the thermodynamic binding constants of a sulfonamide (SA) inhibitor to bovine carbonic anhydrase II (CA) and metal mutated variants (M-CAs) were evaluated. 1-(4-Aminosulfonylphenylazo)-2-naphthol-6,8-disulfonate was used as the SA in the electrophoretic buffer for ACE. The Scatchard analysis of the dependence of the electrophoretic mobility of native CA on the SA concentration provided the binding constant to be K_b = (2.29 ± 0.05) × 10⁶ M^{- 1} (at pH 8.4, 25 °C). On the other hand, apoCA showed far smaller value [K_b = (3.76 ± 0.14) × 10² M^-1], suggesting that the coordination of SA to the Zn^II center controlled the binding thermodynamics. The ACE of M-CAs showed the same behaviors as native CA but with different K_b values. For example, Co-CA adopting the same tetrahedral coordination geometry as native CA exhibited the largest K_b value [(2.55 ± 0.05) × 10⁶ M^-1] among the M-CAs. In contrast, Mn- and Ni-CA, which adopted the octahedral coordination geometry, had K_b values that were about two orders of magnitude lower. Because the hydrophobic cavity of CA around the active center pre-organized the orientation of SA, thereby fixing the ligating NH^- moiety to the apex of the tetrahedron supported by three basal His₃ of CA, metals such as Zn and Co at the center of M-CA gave the most stable CA-SA complex. However, pre-organization was not favored for octahedral geometry. Thus, pre-organization of SA was the key to facilitating the tetrahedral coordination geometry of the Zn^II active center of CA.